At the moment, the Graphics module only supports a fully transparent color. It would be nice if the unused 7 bits (on a 32 bit platform) of any color were used to hold transparency information. One could even provide a full (8 bits) alpha channel on 64 bits platforms in a platform independent way if one requires the colors to be declared through

val rgba : int -> int -> int -> int -> color

Also, an interface with int32 bigarrays would be nice for 64 bits platforms where images take up twice as much space as they really need.